Molecular mechanisms of propyne oxidation on the Pt(111) surface: In situ soft X-ray studies in pressures of oxygen

Oxidation of preadsorbed propyne has been characterized on the Pt(111) surf ace in oxygen at pressures up to 0.009 Torr using fluorescence yield ultras oft X-ray adsorption methods above the carbon K edge. A combination of temp erature-programmed reaction spectroscopy (TPRS) and temperature-programmed fluorescence yield near edge spectroscopy (TP-FYNES) experiments indicating that similar oxidation pathways occur both for coadsorbed oxygen and press ures of oxygen. Soft X-ray spectroscopy indicates that propyne's pi system adsorbs nearly parallel to this surface with a saturation coverage of 1.45 x 10(15) C atoms/cm(2). Oxidation of small propyne coverages with coadsorbe d oxygen results in simultaneous CO2 and H2O peaks at 320 and 420 K, as see n in TPRS. Oxidation of higher propyne coverages with coadsorbed oxygen res ults in a broad oxidation peak over the 350-420 K temperature range. Oxidat ion of a saturated propyne monolayer in oxygen pressures (TP-FYNES) results in a rapid decrease in carbon coverage over the same temperature range, su ggesting similar mechanisms. Isothermal oxidation in oxygen atmospheres ind icates that propyne oxidation is first-order in propyne coverage and has an activation energy of 17 kcal/mol for high coverages. Deviations from first -order behavior suggest that a second process may become important at lower coverages in oxygen atmospheres. Regardless of coverage and initial condit ions, both TPRS and TP-FYNES indicate oxydehydrogenation and skeletal oxida tion occur simultaneously and the oxidation proceeds with a fixed C3H4 stoi chiometry. Taken together, these results give a molecular picture of propyn e oxidation on the Pt(111) surface.